I have a expantion tank on a forces hot water gas furnace that keeps filling with water. I replaced the last one with a new tank and it still fills with water. Does anyone know why and what can I do to prevent this. After it fills with water there is no expantion room and then the blowby valve gets intermittently released.

What type of tank do you have ? A standard type or a bladder tank?

On a standard tank this is usually caused by to little or to much air in the tank. usually draining the tank and then putting it back in service will fix the problem. Now if there is a air leak on any fitting above the hot water storage area of the expansion tank then it will loose its air charge.

On a bladder tank (you can tell this style from the air charger scharader fitting on the bottom of the tank) the bladder goes bad and the tank fills up. Those you have to replace since draining them does no good.

Your fill valve could be defective if it is allowing the pressure in the system to go above the 12 PSI or so level. This will cause the expansion tank to fill with to much water.

This is what a bladder tank looks like.

Thanks HVAC1000. The picture you show is the same that I have. The new one filled with water. I did not check the pressure in it before I installed it. I thought they came charged. I will replace it again and check the pressure and if it fills again I will need to get someone more qualified that I.

Thanks

Usually they are charged with 12 pounds from the factory but if you put a pressure guage on it you will let out some of the charged air so be careful. My men carried a small manual air pump in the trucks for this purpose.

Usually a bladder tank lasts for a good while but system design and tank placement on return or supply line can make a big difference in how long they actually last.

Trick of the day

Your diaphram tank should be charged to the same pressure as the boiler operating pressure. In most cases it is about 12 pounds BUT I have seen higher pressures on some systems.